Cystic fibrosis (CF) is the most common autosomal recessive disorder of Caucasians. The gene which is defective in human CF patients has recently been isolated and characterized. This gene encodes a putative transmembrane protein that may play a role in controlling ion transport across epithelial membranes, and has consequently been termed the cystic fibrosis ion transport regulator (CFTR). Detailed analysis of the human CFTR gene has revealed a single mutation which is present in approximately 70% of CF patients. All CF patients examined to date have some modification of this gene, indicating that defects in CFTR are responsible for the clinical manifestations of CF. It is not known how the defects in CFTR result in the clinical manifestations observed in CF patients. The goal of this project is to create an animal model for CF which can be used to study the basic defect of this disease, and to examine the role of CFTR in the normal and disease state. Detailed analyses of the basic defect in CF patients is very difficult due to an inadequate supply of matched normal and affected tissue, and testing and evaluation of new treatments in human patients is also difficult. The proposed research involves producing a mouse model for CF which contains a mutation in the mouse CFTR gene that is analogous to the defect which has been detected in the majority of human CF patients. This production of an animal model for CF utilizes recent advances in gene targeting (homologous recombination) and manipulations of mouse embryonic stem cells. Briefly, a piece of the mouse CFTR gene will be isolated, and a mutation which is analogous to the one found in many human CF patients will be introduced into the mouse gene. The end product of this procedure is the production of mice which can be used to establish heterozygous lines having one normal and one mutant CFTR gene; these animals can then be mated together to produce homozygous animals that have the gene defect which is analogous to the majority of human CF patients. The availability Of an appropriate animal model for CF will allow investigators to easily examine the physiological effects of a defective CFTR gene, as well as providing a model for examining new treatment regimes. The proposed research will also provide detailed information concerning the expression of the normal mouse CFTR gene.